Copter: bug fix for auto_armed logic

Zero throttle when switching into an auto flight mode would cause the
motors to stop.
AP_Motors: remove auto_armed

redundant flag that was already held in the main code which is where it
belongs.
Copter: remove unused waypoint_valid function
Copter: remove unused nav_ok and alt_sensor_flag
AP_GPS: fixed uninitialied variable bugs found with valgrind
AP_InertialNav: initialise _gps_last_time

found with valgrind
AP_Scheduler: fixed _last_run initialisation

another valgrind bug
Copter: remove unused throttle modes

These were useful for testing the alt-hold when it was first introduced
but now they just add complexity
Copter: resolve compiler warning
Copter: remove unnecessary cast to float in Log
Copter: shorted INAV dataflash log message length

Velocities are now output in WPNav message
Changed instructions for loading fimrware onto PX4
Merge pull request #206 from Craig3DRobotics/master

Changed Instructions for uploading firmware to PX4
AP_Param: fixed assignment handling and const

this fixes radio_min = radio_max = radio_trim = 0; in RC_Channel
AP_Param: added ops needed for ArduCopter build
AC_WPNav: improved comments

Made SPEEDZ a parameter although it's not yet used
AC_WPNav: add climb and descent params, dynamic leash length
Copter: auto climb and descent params removed

These params now reside in the AC_WPNav library
Copter: remove unused AUTO_VELZ defaults

These have now moved to AC_WPNav
Copter: add WP_YAW_BEHAVE parameter

Allows user to control whether nose points towards next waypoint and/or
home when RTLing.
Replaces YAW_OVR_BEHAVE which had a very similar purpose.  Almost nobody
ever knew about or used this parameter.